The underlying hypothesis of this grant is that alterations in costimulatory and activation pathways lead to a lower T cell activation threshold and thus to great susceptibility to autoimmune disease. Our efforts will focus on regions identified by the first whole genome admixture association study which led to the discovery of a 4 MB locus on chromosome 1 with an admixture LOD score of 6.3;this admixture locus contains the co-stimulatory molecules: CD2, CD58 (LFA-3), CD101 and B7-h4. Fine association mapping indicated that allelic variation in CD58 is most strongly associated with susceptibility to MS. This finding is of particular interest because of the recently demonstrated role for CD58 engagement of CD2 in the generation of regulatory Tr1 and CD4+CD25high cells. Moreover, genetic variation in CD101 and B7-h4 also exhibit suggestive evidence of association to MS susceptibility. These results parallel those of Wicker &Todd who defined the murine T1D susceptibility loci Idd10, a locus containing the CD101/B7-H4 genes, and Idd18.2 containing the CD2 gene. To examine the hypothesis that allelic variation of costimulatory genes CD58, CD101 &CTLA-4 affect the expression of these and other molecules that are associated with susceptibility to MS, we will in aim 1 identify the risk-associated genetic variation of CD58 in our MS patients by genotyping all publicly available SNPs within this gene followed by resequencing the implicated segment of the CD58. In the second aim, we will determine whether there are additional MS risk alleles of co-stimulation gene variants in the ADMS1 locus and in close collaboration with Todd, determine whether the T1D CTLA4 risk haplotype confers increased risk for MS. The last two aims explore possible functional effects of the risk alleles that are characterized in Aims 1 and 2. These characterizations of CD58 and CTLA4 are driven by the important roles of these molecules in immune regulation in addition to our genetic analyses. In aim 3 we will determine whether soluble CD58 and CTLA-4 levels are altered in MS independently of the effect of the different haplotypes present in the patient and control populations. In close collaboration with Wicker, we will investigate whether the T1D risk haplotype of CTLA4 correlates with decreased soluble CTLA4 and increased CD101 expression on the CD8+CD45RA- cells of subjects with MS. In aim 4 we will determine whether there are differences in the isoform distribution of CD58 between MS cases and control subjects that are independent of the effect of the different haplotypes of CD58. We hypothesize that allelic variants of CD58, CTLA-4 or CD101 dictate subtle but measurable changes in expression and function that are linked to a lower signaling threshold and susceptibility to autoimmune disease.